This is an application for funds to support a five-year program of research to investigate the genetic and environmental causes of Specific Language Impairment (SLI) in children in a large sample of twins and single born children ages 2 to 8 years, and their families. Building on prior work that established a grammatical phenotype in young children, the proposed studies will investigate detailed quantitative language phenotypes, as a function of known risk factors: multiple birth/twinning, late-talker status, limited home resources (e.g., low SES), positive history of family affectedness, and lower levels of language performance among family members. The project substrate is an available participant pool of 1,879 singleton children with known health and developmental histories who will be 6-7 years of age in mid 2002, and a twin participant pool that will accumulate to 720 twin pairs over the course of the study, ages 2-6 years. These large populations are available in Perth, Australia, as the result of previous health studies. In addition to the proband assessments, 824 members of the nuclear families of affected twin and singleton children will participate in the protocols. The research design proceeds from unvaried comparisons of groups of affected and control children, to multivariate analyses to evaluate the latent structures of the language phenotype, and to provide estimates for the relative effects of genetic factors, and shared and non-shared environmental effects. The outcomes will be highly relevant for the identification of children at clinical risk for language acquisition and the estimation of possible genetic, as well as environmental, effects on grammatical and vocabulary development in affected children.